1. Field of the Invention
The present invention relates to a light guide member, an illumination device, and an image reading apparatus using them and, more particularly, to an apparatus for illuminating an original with a linear light beam while optically illuminating, guiding the light beam from the original onto a light-receiving surface to form an original image, and reading the original image.
2. Related Background Art
FIG. 1 is a view for explaining a prior art (Japanese Patent Application Laid-Open No. 9-61633). A light beam from a light-emitting member 41 is incident on a columnar light guide member 45 from its end portion and propagates in the columnar light guide member 45. The light beam illuminating a light-diffusing portion 43 is diffused, and part of the diffused light beam emerges downward from a light emergent portion 44 in FIG. 1.
FIG. 2 is a view for explaining an optical path when the prior art in FIG. 1 is applied to an image reading apparatus. Reference numeral 46 in FIG. 1 denotes an imaging lens of an image reading system; and 47, a transmission original.
In this apparatus, light beam from the light-emitting member 41 enters into the columnar light guide member 45 from a light incident surface 42 at the end portion of the light guide member 45 and propagates through the light guide member 45. The light beam then emerges outside the light guide member 45 from the light emergent portion 44 formed on the outer surface along the longitudinal direction. The exit light beam illuminates the transmission original 47. An image of the transmission original 47 illuminated with the light beam is formed on the surface of a light-receiving element (not shown) by the imaging lens 46, thereby reading the image.
Generally, in an illumination device using a light guide member, of all the light beams repeating total reflection in the light guide member, light beam that does not satisfy the total reflection condition on the inner wall of the light guide member and emerges outside the light guide member is used to illuminate the original. When a reflection/diffusion surface is formed on the surface opposing the light emergent surface, an amount of light beam directly reflected by a reflection/diffusion surface and emerging outside from the light emergent portion without total reflection is relatively large in all the light beam incident on the light guide member.
In particular, as in the conventional case, when the light-diffusing surface is a saw-toothed reflection surface and the saw-toothed projections are formed on the reflection surface at an equal pitch, part of the light beam directly emerging from the light emergent portion does not enter the pupil of the imaging lens, resulting in low illumination efficiency.